Adjacent channel leakage ratio (ACLR) is the ratio of power measured in an adjacent channel to transmitted power. In the case of wideband code division multiple access (W-CDMA), both the transmitted power and the adjacent channel power are measured with a filter that has a root raised-cosine (RRC) filter response with rolloff of a=0.22 and a bandwidth equal to the chip rate.
ACLR is one of the most important measurements in the wireless communication industry. Performing ACLR with measurement precision (or repeatability) and speed are important factors when minimizing manufacturing and testing costs for wireless communication devices.
In the case of W-CDMA (also called spread spectrum) manufacturing, the 3GPP specifications require measurements of the power ratios of two adjacent channels (+/−5 MHz offset from the carrier frequency) and two alternative channels (+/−10 MHz offset from the carrier frequency) over the carrier channel power. For ACLR, sufficient dynamic range is required, for example, −65 decibels referred to the carrier (dBc) for the case of the base station manufacturing test. In order to achieve higher measurement accuracy for channel power, longer observation time is needed. This typically results in lower measurement speed.
For example, when performing ACLR using an Agilent E4406A Vector Signal Analyzer or using an Agilent PSA series High-Performance Spectrum Analyzer (e.g., E4443A, E4445A, E4440A, E4446A or E4448A, PSA Series Spectrum Analyzer), fast Fourier transform (FFT) or swept mode is used to acquire a spectrum trace. An RRC filter is applied in the frequency domain. The spectrum trace is integrated to determine channel power at each offset (including the carrier channel). All the mathematical calculations are typically performed on a host central processing unit (CPU).
For example, using an Agilent PSA series High-Performance Spectrum Analyzer in swept mode, narrow resolution bandwidth (RBW) is required to make sure the spectrum trace has enough resolution to present the RRC filter shape. Also, long enough sweep time is needed to achieve the measurement accuracy.
For example, using an Agilent E4406A Vector Signal Analyzer or using an Agilent PSA series High-Performance Spectrum Analyzer in FFT mode, three dominant factors impact measurement performance. First, the local oscillators (LOs) used for testing must be re-tuned several times to accommodate the ACLR requirement to analyze around 25 MHz of bandwidth. Each LO retuning takes about 5 milliseconds (ms), which is a relatively long time. Second, the speed of the FFT and RRC filter operation is limited by the host CPU. Third, data transfer from a data capture memory to the host is time consuming.